Re: [PATCH v4 6/6] KVM: arm64: Add auto HDBSS enable/disable on dirty logging change

[Tian Zheng]

On 7/13/2026 10:50 PM, Leonardo Bras wrote:
> On Thu, Jul 09, 2026 at 06:40:26PM +0800, Tian Zheng wrote:
> > From: eillon <yezhenyu2@xxxxxxxxxx>
> >
> > HDBSS buffers store per-page dirty state after the stage-2 page tables
> > have been split down to page granularity (chunk_size == PAGE_SIZE).
> chunk_size != PAGE_SIZE now, but that should change as well :)
>
> > When chunk_size == 0 the kernel may lazily skip splitting block mappings,
> > leaving the page table coarser than what HDBSS expects. Therefore,
> > enabling HDBSS requires disabling lazy split so that all block mappings
> > are eagerly broken down before the buffer starts recording.
> (See cover letter reply)
>
> > Add VM-level HDBSS enable/disable support. When dirty logging is
> > enabled on any memslot, HDBSS is automatically enabled. When dirty
> > logging is disabled on all memslots, HDBSS is automatically disabled.
> >
> > This includes:
> > - kvm_arm_enable_hdbss_global() to enable HDBSS for all vCPUs
> > - kvm_arm_disable_hdbss_global() to disable and free HDBSS buffers
> > - kvm_arm_hdbss_on_dirty_logging_change() for auto enable/disable
> > - kvm_arch_destroy_vm() cleanup path
> > - kvm_arch_commit_memory_region() integration
> >
> > Signed-off-by: Eillon <yezhenyu2@xxxxxxxxxx>
> > Signed-off-by: Tian Zheng <zhengtian10@xxxxxxxxxx>
> > ---
> >   arch/arm64/include/asm/kvm_dirty_bit.h |   2 +
> >   arch/arm64/kvm/arm.c                   |   8 ++
> >   arch/arm64/kvm/dirty_bit.c             | 105 +++++++++++++++++++++++++
> >   arch/arm64/kvm/mmu.c                   |   3 +
> >   4 files changed, 118 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/kvm_dirty_bit.h b/arch/arm64/include/asm/kvm_dirty_bit.h
> > index 4b28000e972f..a4cda8cdab24 100644
> > --- a/arch/arm64/include/asm/kvm_dirty_bit.h
> > +++ b/arch/arm64/include/asm/kvm_dirty_bit.h
> > @@ -23,5 +23,7 @@ int kvm_arm_vcpu_alloc_hdbss(struct kvm_vcpu *vcpu, unsigned int order);
> >   void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu);
> >   void kvm_flush_hdbss_buffer(struct kvm_vcpu *vcpu);
> >   int kvm_handle_hdbss_fault(struct kvm_vcpu *vcpu);
> > +void kvm_arm_hdbss_on_dirty_logging_change(struct kvm *kvm, int nr_memslots_logging);
> > +void kvm_arm_disable_hdbss_global(struct kvm *kvm);
> >
> >   #endif /* __ARM64_KVM_DIRTY_BIT_H__ */
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 566953a4e23a..536d94799ba8 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -317,6 +317,14 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
> >   	if (is_protected_kvm_enabled())
> >   		pkvm_destroy_hyp_vm(kvm);
> >
> > +	/*
> > +	 * Userspace may destroy the VM without disabling dirty logging,
> > +	 * so the auto-disable path is never reached. Force disable HDBSS
> > +	 * here to ensure vCPU buffers are freed and prevent memory leaks.
> > +	 */
> > +	if (kvm->arch.enable_hdbss)
> > +		kvm_arm_disable_hdbss_global(kvm);
> > +
> >   	kvm_uninit_stage2_mmu(kvm);
> >   	kvm_destroy_mpidr_data(kvm);
> >
> > diff --git a/arch/arm64/kvm/dirty_bit.c b/arch/arm64/kvm/dirty_bit.c
> > index 002366337637..c5bf866c23ef 100644
> > --- a/arch/arm64/kvm/dirty_bit.c
> > +++ b/arch/arm64/kvm/dirty_bit.c
> > @@ -112,3 +112,108 @@ int kvm_handle_hdbss_fault(struct kvm_vcpu *vcpu)
> >   		return -EFAULT;
> >   	}
> >   }
> > +
> > +static unsigned int hdbss_auto_select_order(struct kvm *kvm)
> > +{
> > +	unsigned long npages = 0;
> > +	struct kvm_memory_slot *memslot;
> > +	int bkt;
> > +
> > +	kvm_for_each_memslot(memslot, bkt, kvm_memslots(kvm))
> > +		npages += memslot->npages;
> > +
> > +	if (npages <= 16384)
> > +		return 0;
> > +	else if (npages <= 262144)
> > +		return 3;
> > +	else if (npages <= 4194304)
> > +		return 6;
> > +	else
> > +		return 9;
> > +}
> IIUC you are counting the amount of pages the VM has, and based on that
> allocating a size for the HDBSS buffer.
>
> A few notes here:
> - It's not really nice to use magic numbers around like this. If you
>    actually want to use it, then use stuff like SZ_16K, SZ_256K, SZ_4M and
>    so on.
> - You are returning magic numbers as well, why is it 0, 3, 6, or 9 here?
>    It only makes sense if the person is reading HDBSSBR_EL2 documentation,
>    which should not be necessary at this point. That's one reason I
>    recommended to using sizes. If that was really the best way to use it,
>    I would recommend using the defines that we get from sysreg, and you
>    actually used before to set the maximum order on a previous patch.
> - Also, if you can return only valid values here, why do you check against
>    the maximum value in that previous patch?
> - Also, are you using some undisclosed rule here? On 'order 0' the
>    meanining is 4KB, which translate to 512 HDBSS entries. Why are you using
>    it for any value under 16K? Same for 3-32KB-4kEntries you use for under
>    256K pages (and so on). If you are assuming a logical rule such as
>    'N pages would be ok with N/32 entries' it has to be described here at
>    least.
> - Not sure VM size is the best way of doing that, since it will depend
>    more on the dirtying rate than the actual size, and most VMs would just
>    use the biggest size (4M x 4K pages is just 16GB). For instance with
>    dirty_ring we can use the dirty_ring.size as a better option.
>    (I know this is a hard one to estimate when using dirty-bitmap, though)
>
> > +
> > +/*
> > + * Enable HDBSS for all vCPUs in the VM.
> > + *
> > + * Called from kvm_arm_hdbss_on_dirty_logging_change() which is invoked
> > + * by kvm_arch_commit_memory_region() under kvm->slots_lock.
> > + *
> > + * If buffer allocation fails, HDBSS remains disabled and dirty tracking
> > + * falls back to the traditional software-based approach (PTE write-protect
> > + * + software dirty marking). This does not affect correctness; dirty
> > + * logging remains functional without HDBSS.
> > + */
> > +static int kvm_arm_enable_hdbss_global(struct kvm *kvm)
> > +{
> > +	int err;
> > +	unsigned long i;
> > +	unsigned int order;
> > +	struct kvm_vcpu *vcpu;
> > +
> > +	if (!system_supports_hdbss())
> > +		return 0;
> > +
> > +	if (kvm->dirty_ring_size) /* Don't support HDBSS in dirty ring mode */
> > +		return 0;
> > +
> > +	if (kvm->arch.enable_hdbss) /* Already On */
> > +		return 0;
> > +
> > +	/* Turn it on */
> > +	order = hdbss_auto_select_order(kvm);
> > +	kvm_for_each_vcpu(i, vcpu, kvm) {
> > +		err = kvm_arm_vcpu_alloc_hdbss(vcpu, order);
> > +		if (err)
> > +			goto error_alloc;
> > +	}
> > +
> > +	kvm->arch.enable_hdbss = true;
> > +	kvm->arch.mmu.vtcr |= VTCR_EL2_HD | VTCR_EL2_HDBSS | VTCR_EL2_HA;
> > +
> > +	/*
> > +	 * We should kick vcpus out of guest mode here to load new
> > +	 * vtcr value to vtcr_el2 register when re-enter guest mode.
> > +	 */
> > +	kvm_for_each_vcpu(i, vcpu, kvm)
> > +		kvm_vcpu_kick(vcpu);
> > +
> > +	return 0;
> > +
> > +error_alloc:
> > +	kvm_for_each_vcpu(i, vcpu, kvm)
> > +		if (vcpu->arch.hdbss.base_phys)
> > +			kvm_arm_vcpu_free_hdbss(vcpu);
> > +
> > +	pr_warn_once("kvm: failed to allocate HDBSS buffers (order=%u), "
> > +		     "falling back to software dirty tracking\n", order);
> > +	return -ENOMEM;
> > +}
> > +
> > +void kvm_arm_disable_hdbss_global(struct kvm *kvm)
> > +{
> > +	unsigned long i;
> > +	struct kvm_vcpu *vcpu;
> > +
> > +	if (!kvm->arch.enable_hdbss) /* Already Off */
> > +		return;
> > +
> > +	/* Turn it off */
> > +	kvm->arch.mmu.vtcr &= ~(VTCR_EL2_HD | VTCR_EL2_HDBSS | VTCR_EL2_HA);
> > +
> > +	kvm_for_each_vcpu(i, vcpu, kvm)
> > +		kvm_arm_vcpu_free_hdbss(vcpu);
> > +
> > +	kvm->arch.enable_hdbss = false;
> > +}
> > +
> Okay, say the user requested it to be disabled, you change the global vtcr,
> then free the hdbss on every vcpu.
>
> But the vcpus are still running, and since they will only disable this when
> they go out of the guest, then in again, HDBSS will still be running,
> right?
>
> If some page gets dirty in the between, would not the HDBSS try to write to
> the already loaded buffer adress, and write to memory that have already
> been freed here?
>
>
> > +void kvm_arm_hdbss_on_dirty_logging_change(struct kvm *kvm, int nr_memslots_logging)
> > +{
> > +	/*
> > +	 * Called from kvm_arch_commit_memory_region() under kvm->slots_lock.
> > +	 * All state transitions are serialized by slots_lock.
> > +	 */
> > +	if (nr_memslots_logging > 0 && !kvm->arch.enable_hdbss)
> > +		kvm_arm_enable_hdbss_global(kvm);
> > +	else if (nr_memslots_logging == 0 && kvm->arch.enable_hdbss)
> > +		kvm_arm_disable_hdbss_global(kvm);
> > +}
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 949fb895add6..484f48dae000 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -2588,6 +2588,9 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> >   {
> >   	bool log_dirty_pages = new && new->flags & KVM_MEM_LOG_DIRTY_PAGES;
> >
> > +	kvm_arm_hdbss_on_dirty_logging_change(kvm,
> > +		atomic_read(&kvm->nr_memslots_dirty_logging));
> > +
> >   	/*
> >   	 * At this point memslot has been committed and there is an
> >   	 * allocated dirty_bitmap[], dirty pages will be tracked while the
> > --
> > 2.33.0
> Okay, reading the above I remembered something really complicated:
> We can't really enable HDBSS partially if we start with DBM set for all
> pages. Once we enable HDBSS wit will track changes for all memslots.
>
> The only way to enable it partially would be to set DBM during the
> dirty-bit tracking, which I recall being complicated for some reasons.
>
> Well, we have to think about the overall strategy before a next version.
>
> Thanks!
> Leo


Hi, Leo


One thing just clicked for me: we don't actually need per-memslot DBM 
control.



PML on x86 works the same way — D bit set on all writable SPTEs 
unconditionally,

buffer records everything. The filtering happens in software at flush time:

vmx_flush_pml_buffer() -> kvm_vcpu_mark_page_dirty() -> 
mark_page_dirty_in_slot(),

where kvm_slot_dirty_track_enabled() discards non-tracked memslots.


Our HDBSS does exactly the same: global DBM, buffer records all writes,

kvm_flush_hdbss_buffer() -> mark_page_dirty_in_slot() filters at flush time.

So I'm questioning the premise — why should HDBSS be held to a different 
standard

than PML? If overhead becomes a real problem, we can revisit per-memslot 
later.

For v5, I think global DBM is fine.

So, I think we just need to focus on how to correctly handle DBM for 
huge pages.